1. Field of the Invention
The present invention relates to an electrostatic actuator formed by using the micromachine or MEMS (Micro-Electro-Mechanical Systems) technique and, more particularly, to a switch, variable capacitor, or the like using the electrostatic actuator.
2. Description of the Related Art
A MEMS switch using an electrostatic actuator is disclosed in, e.g., U.S. Pat. No. 6,483,395 or MEMS SWITCH LIBRARY pp. 130-139. To turn on the MEMS switch, a potential difference is applied between upper and lower electrodes of the electrostatic actuator so that the electrostatic attraction between the upper and lower electrodes exceeds the spring force of a beam.
The driving voltage of this electrostatically driven MEMS switch is as high as 20V or more. A charge pump is necessary because this driving voltage is higher than the power supply voltage of a portable terminal system. The charge pump is disadvantageous for a portable system because the circuit has a large chip area and high power consumption. In addition, noise generated by the charge pump may cause an operation error of a wireless circuit.
The driving voltage can be lowered by decreasing the spring constant of the electrostatic actuator. However, a low driving voltage weakens the force that brings the upper and lower electrodes into contact with each other, i.e., the contact force. As a consequence, the contact resistance of the switch increases. Also, if the spring constant is small, the separation force weakens. This readily causes a failure by which the upper and lower electrodes are not separated but kept in contact with each other, i.e., a so-called stiction failure.
The above-mentioned problems of the MEMS switch similarly exist in a MEMS variable capacitor. That is, if the spring constant is decreased in order to lower the driving voltage, the contact force weakens, so no high capacitance ratio can be obtained any longer. Also, a stiction failure readily occurs as in the switch.